Japanese cedar (Cryptomeria japonica) ash was used as a natural activating agent and also a precursor to prepare activated carbon (AC) using various ash to water ratios (0.20, 0.25, and 0.33 wt%), activation temperatures (750, 850, and 950 A degrees C), and activation durations (0.5, 1.0, and 1.5 h). The yields, iodine value, and methylene blue adsorption value of Japanese cedar AC were in the ranges of 27.3-30.7 %, 953.5-1150.9, and 695.1-1368.0 mg/g. As the ash to water ratio increased, the BET specific surface area and total pore volume of the AC increased. Japanese cedar AC prepared using an ash to water ratio of 0.33 wt%, an activation temperature of 850 A degrees C, and an activation duration of 1.0 h exhibited the highest BET specific surface area (1430 m(2)/g) and total pore volume (0.74 cm(3)/g). The nitrogen adsorption-desorption isotherms of the Japanese cedar AC were classified as type IV, indicating the presence of microporous and mesoporous structures, according to the Bruauer, Deming, Deming, and Teller classification. Therefore, Japanese cedar ash with a suitable ash to water ratio can serve as an excellent natural activating agent for preparing AC because a high BET specific surface area can be obtained.